There are known item sorting systems where a wheel drive cart is configured to tow a plurality of loading carts along a closed path defined by a pair of rails where at least one item feeding station arranged along the rails is configured to load items onto the carts and at least one unloading station arranged along the rails is configured to receive items unloaded from the carts.
For example, each cart can support a powered belt or a tray that can be inclined with respect to a rotation axis. In the first case, the item placed on the powered belt is actively discharged sideways at the unloading station, while in the case of using a tray this can be opportunely tilted in the unloading step so that the item resting on the tray falls by gravity.
Non-limitative examples of transportable/sortable items are:                luggage handled by an airport-type luggage sorting system;        mail items (bundles of mail items, parcels, boxes, packets in general, etc.) handled by a mail sorting system;        packaged products (boxes for example) picked from a warehouse and sent to a delivery system.        
The wheel drive cart typically comprises a supporting frame provided with a first and a second idler wheel arranged on opposite sides of the frame and adapted to rest on respective first and second resting surfaces of the first and second rail, which face upwards in use. The wheel drive cart further comprises at least one first contrast idler wheel carried by the frame and adapted to abut on a first inner side of the first rail facing a second inner side of the second rail on which a second contrast wheel abuts in order to keep the wheel drive cart positioned between the first and the second rail during movement of the wheel drive cart along the rails.
Different technologies are used to provide the motion of the wheel drive cart. For example, wheel drive carts could be provided with high-energy permanent magnets (typically rare earths) having the function of a rotor and stator coils arranged along the rail that are powered in sequence to obtain a moving magnetic field, which by interacting with the permanent magnets causes movement of the cart along the track.
Using opportune mechanical solutions, a constant air gap of a few millimetres is guaranteed between the carts and the stator coils, so that the magnetic field generated by the permanent magnets can be detected and used to control the powering of the coils.
The main advantage of this solution is represented by the total absence of contact between the parts that form the linear electric motor which drives the cart; for this reason, maintenance is extremely low, as there is virtually no wear on the parts in movement.
The limits of the above-described magnetic solution are instead represented by the high costs associated with the permanent magnets and the coil system, and by the high mechanical rigidity required to maintain the predetermined air gap under all conditions of motion conditions along the track.
Other solutions of the mechanical type envisage the use of groups of pinch wheels driven by motor reducers and arranged along the path beneath the rails. These pinch wheels are configured to pinch opportunely shaped fins rigidly constrained under the carts and thus impart acceleration to the carts.
In particular, each group of pinch wheels forms a driving force zone that contributes to generating part of the overall propelling force with a tractive load that is thus spread over several carts along the rails.
The main drawbacks of this mechanical type of solution are represented by the need for careful and constant maintenance of the pinch groups, and by the risk of carts crashing against the pinch group and the track in the event of jamming anomalies. Furthermore, the distribution and electrical wiring of the pinch groups along the track are often problematic.
In addition, solutions of the electromechanical type have been proposed in which the wheels of the cart have been motorized by using electric motors carried on the cart. These solutions envisage that the electric motor cooperates with a complex mechanical drive train for transfer of motive power from the motor to the drive wheels and the reduction of the number of revs of the electric motor. Use of a mechanical drive train entails a series of problems as it is heavy, bulky and has intrinsic reliability problems as, in the case of failure within the drive train, it is practically impossible to mechanically disconnect the transmission from the electric motor while the cart is in movement.
None of the proposed solutions therefore satisfies the requirements requested for sorting systems.
These requirements are particularly pressing as the market asks for sorting and transporting systems capable of operating almost continuously, up to twenty-three hours a day, guaranteeing a redundancy level for the plant of around 99% even in the event of a cart failure.
In some specific applications, for example of the postal type, high performance in terms of speed is also required to ensure an efficient high-capacity sorting process (up to 30000 items per hour) and a low margin of error (less than 1/20000 items erroneously sorted).
For example, in some applications it is necessary for the sorter's cart train to move along a closed loop at a high constant speed, for example 2.5 m/s.
The need is therefore felt for providing a wheel drive cart that satisfied the above-mentioned requirements by overcoming the problems of the currently known magnetic, mechanical and electromechanical solutions.